Lecture 2 (2.1-2

ECE321 Electronics I: Lecture 2

Chapter 2 Solid-State Electronics (Sections 2.1-2.6) Microelectronic Circuit Design Richard C. Jaeger Travis N. Blalock

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Chap 2 - 1

Chapter Goals (Lectures 2 & 3) • Explore semiconductors and discover how engineers control semiconductor properties to build electronic devices. • Characterize resistivity of insulators, semiconductors, and conductors. • Develop covalent bond and energy band models for semiconductors. • Understand band gap energy and intrinsic carrier concentration. • Explore the behavior of electrons and holes in semiconductors. • Discuss acceptor and donor impurities in semiconductors. • Learn to control the electron and hole populations using impurity doping. • Understand drift and diffusion currents in semiconductors. • Explore low-field mobility and velocity saturation. • Discuss the dependence of mobility on doping level. Jaeger/Blalock 4/15/07

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Chap 2 - 2

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The Kilby Integrated Circuit (1950s) Semiconductor die

Active device

Electrical contacts Jaeger/Blalock 4/15/07

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Chap 2 - 3

Solid-State Electronic Materials • Electronic materials fall into three categories: – Insulators – Semiconductors – Conductors

Resistivity (ρ) > 105 Ω-cm 10-3 < ρ < 105 Ω-cm ρ < 10-3 Ω-cm

• Elemental semiconductors are formed from a single type of atom, typically Silicon. • Compound semiconductors are formed from combinations of column III and V elements or columns II and VI. • Germanium was used in many early devices. • Silicon quickly replaced germanium due to its higher bandgap energy, lower cost, and is easily oxidized to form silicon-dioxide insulating layers. Jaeger/Blalock 4/15/07

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Chap 2 - 4

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Semiconductor Materials (cont.) Semiconductor

Bandgap Energy EG (eV)

Carbon (diamond)

5.47

Silicon

1.12

Germanium

0.66

Tin

0.082

Gallium arsenide

1.42

Gallium nitride

3.49

Indium phosphide

1.35

Boron nitride

7.50

Silicon carbide

3.26

Cadmium selenide

1.70

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Chap 2 - 5

Covalent Bond Model (shared electrons) Valence = 4; “stable-8” configuration

Silicon diamond lattice unit cell.

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Corner of diamond lattice showing four nearest neighbor bonding. Microelectronic Circuit Design McGraw-Hill

View of crystal lattice along a crystallographic axis.

Chap 2 - 6

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Intrinsic (pure) Silicon Covalent Bond Model (cont.)

Near absolute zero, all bonds are complete. Each Si atom contributes one electron to each of the four bond pairs. Jaeger/Blalock 4/15/07

Increasing temperature adds energy to the system and breaks bonds in the lattice, generating electron-hole pairs.

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Chap 2 - 7

Intrinsic Carrier Concentration ni = electron density = pi = hole density • The density of carriers in a semiconductor as a function of temperature and material properties is:

⎛ E ⎞ n i2 = BT 3 exp⎜− G ⎟ cm-6 ⎝ kT ⎠

• EG = semiconductor bandgap energy in eV (electron volts) • k = Boltzmann’s constant, 8.62 x 10-5 eV/K • T = absolute termperature, K • B = material-dependent parameter, 1.08 x 1031 K-3 cm-6 for Si • Bandgap energy is the minimum energy needed to free an electron by breaking a covalent bond in the semiconductor crystal.

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Chap 2 - 8

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Intrinsic Carrier Concentration (cont.)

Intrinsic carrier density (cm-3)

• Electron density is n (electrons/cm3) and ni for intrinsic material n = ni. • Intrinsic refers to properties of pure materials. • Example 2.1: ni (300K) = 6.73x109 cm-3 ≈ 1010 cm-3 for Si 22 • Approx 5x10 atoms/cm3 means approx 1 in 1013 bonds broken at room temperature (300K)

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Chap 2 - 9

Electron-hole concentrations • A vacancy is left when a covalent bond is broken. • The vacancy is called a hole. • A hole moves when the vacancy is filled by an electron from a nearby broken bond (hole current). • Hole density is represented by p. • For intrinsic silicon, n = ni = p = pi. • The product of electron and hole concentrations is pn = ni2 (Law of Mass Action). • The pn product above holds when a semiconductor is in thermal equilibrium (not with an external voltage applied). Jaeger/Blalock 4/15/07

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Chap 2 - 10

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Drift Current • Electrical resistivity ρand its reciprocal, conductivity σ, characterize current flow in a material when an electric field is applied. • Charged particles move or drift under the influence of the applied field. • The resulting current is called drift current. • Drift current density is j = Qv (C/cm3)(cm/s) = A/cm2 j = current density, (Coulomb charge moving through a unit area) Q = charge density, (Charge in a unit volume) v = velocity of charge in an electric field. Note that “density” may mean area or volumetric density, depending on the context: current density/unit area; charge density/unit volume. Jaeger/Blalock 4/15/07

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Chap 2 - 11

Mobility • At low fields, carrier drift velocity v (cm/s) is proportional to electric field E (V/cm). The constant of proportionality is the mobility, µ: • vn = - µnE and vp = + µpE , where • vn and vp = electron and hole velocity (cm/s), • µn and µp = electron and hole mobility (cm2/V⋅s) • µn = 1350 cm2/V⋅s, µp = 500 cm2/V⋅s (intrinsic Si at 300K) • Hole mobility is less than electron mobility, since hole current is the result of multiple covalent bond disruptions, while electrons can move freely about the crystal. Jaeger/Blalock 4/15/07

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Chap 2 - 12

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Velocity Saturation At high fields, carrier velocity saturates and places upper limits on the speed of solid-state devices.

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Chap 2 - 13

Intrinsic Silicon Resistivity • Given drift current and mobility, we can calculate resistivity: jndrift = Qnvn = (-qn)(-µnE) = qn µnE A/cm2 jpdrift = Qpvp = (+qp)(+µpE) = qp µpE A/cm2 where q = electronic charge = 1.6 x 10-19C jTdrift = jn + jp = q(n µn + p µp)E = σE This defines electrical conductivity: σ = q(n µn + p µp) (Ω⋅cm)-1 Resistivity ρ is the reciprocal of conductivity: ρ = 1/σ (Ω⋅cm) Units: [ρ] = [E]/[j] = [V/cm]/[A/cm2] = Ώ.cm Jaeger/Blalock 4/15/07

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Chap 2 - 14

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Example 2.2: Calculate the resistivity of intrinsic silicon Problem: Find the resistivity of intrinsic silicon at room temperature and classify it as an insulator, semiconductor, or conductor. Solution: • Known Information and Given Data: The room temperature mobilities for intrinsic silicon were given right after Eq. 2.5. For intrinsic silicon, the electron and hole densities are both equal to ni. • Unknowns: Resistivity ρ and classification. • Approach: Use Eqs. 2.8 and 2.9. [σ = q(n µn + p µp) (Ω⋅cm)-1] • Assumptions: Temperature is unspecified; assume “room temperature” with ni = 1010/cm3. • Analysis: Next slide…

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Chap 2 - 15

Example: Calculate the resistivity of intrinsic silicon (cont.) • Analysis: Charge density of electrons is Qn = -qni and for holes is Qp = +qni. Substituting into Eq. 2.8: σ = (1.60 x 10-19)[(1010)(1350) + (1010)(500)] (C)(cm-3)(cm2/V⋅s) = 2.96 x 10-6 (Ω⋅cm)-1 ---> ρ = 1/σ = 3.38 x 105 Ω⋅cm From Table 2.1, intrinsic silicon is near the low end of the insulator resistivity range • Check of Results: Resistivity has been found, and intrinsic silicon is a poor insulator.

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Chap 2 - 16

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Extrinsic (doped) Silicon: Semiconductor Doping • Doping is the process of adding very small well controlled amounts of impurities into a semiconductor. • Doping enables the control of the resistivity and other properties over a wide range of values. • For silicon, impurities are from columns III and V of the periodic table: Group III: B, Al, Ga, In Group V: P, As, Sb Jaeger/Blalock 4/15/07

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Chap 2 - 17

Donor Impurities in Silicon • Phosphorous (or other column V element) atom replaces silicon atom in crystal lattice. • Since phosphorous has five outer shell electrons, there is now an ‘extra’ electron in the structure. • Material is still charge neutral, but very little energy is required to free the electron for conduction since it is not participating in a bond. • Electron is free to drift in applied electric field Jaeger/Blalock 4/15/07

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Chap 2 - 18

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Acceptor Impurities in Silicon • Boron (column III element) has been added to silicon. • There is now an incomplete bond pair, creating a vacancy for an electron. • Little energy is required to move a nearby electron into the vacancy. • As the ‘hole’ propagates, charge is moved across the silicon.

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Chap 2 - 19

Acceptor Impurities in Silicon (cont.)

Hole is propagating through the silicon, indirectly by electron movements. Jaeger/Blalock 4/15/07

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Chap 2 - 20

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Doped Silicon Carrier Concentrations • If n > p, the material is n-type. If p > n, the material is p-type. • The carrier with the largest concentration is the majority carrier, the smaller is the minority carrier. • ND = donor impurity concentration atoms/cm3 NA = acceptor impurity concentration atoms/cm3 • Charge neutrality requires q(ND + p - NA - n) = 0 • It can also be shown that pn = ni2, even for doped semiconductors in thermal equilibrium. Jaeger/Blalock 4/15/07

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Chap 2 - 21

n-type Material • Substituting p = ni2/n into q(ND + p - NA - n) = 0 yields n2 - (ND - NA)n - ni2 = 0. • Solving for n n=

(N D − N A ) ± (N D − N A ) 2 + 4n i2 2

and p =

n i2 n

• For (ND - NA) >> 2ni, n ≅ (ND - NA) ≈ ND if NA = 0.

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Chap 2 - 22

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p-type Material • Similar to the approach used with n-type material we find the following equations:

p=

(N A − N D ) ± (N A − N D ) 2 + 4n i2 2

and n =

n i2 p

• We find the majority carrier concentration from charge neutrality (Eq. 2.10) and find the minority carrier conc. from the thermal equilibrium relationship (Eq. 2.3). • For (NA - ND) >> 2ni, p ≅ (NA - ND) ≈ NA if ND = 0. Jaeger/Blalock 4/15/07

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Chap 2 - 23

Practical Doping Levels • Majority carrier concentrations are established at manufacturing time and are independent of temperature (over practical temp. ranges). • However, minority carrier concentrations are proportional to ni2, (highly temperature dependent). • For practical doping levels, n ≅ (ND - NA) for n-type and p ≅ (NA - ND) for p-type material. • Doping compensation. • Typical doping ranges are 1014/cm3 to 1021/cm3. • Compare 5 x 1022 atoms/cm3. Jaeger/Blalock 4/15/07

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Chap 2 - 24

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End of Lecture 2

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